feat (Db.php) add 'connection.cache' config variable
feat (Db.php) add support to cache or not cache connection in connect() method

This feature allow to use each instances of OracleDb separate connections and as result separate transactions
If 'connection.cache' is false then it oci_new_connect is used, otherwise oci_connect is used, that is caching connection(allow to use 1 connection by lot of instances)